The objectives of this project are to evaluate the extent and metabolic significance of heme and bilirubin compartmentation in the liver, at the level of both the hepatic lobule and individual parenchymal cells. Several different approaches are propossed using recently developed probes of heme and bile pigment metabolism (e.g., isotopically labeled precursors) applied in a variety of experimental systems (e.g., isolated rat liver perfused with a heme-free medium, and isolated hepatocytes). Specifically, attempts will be made to define the metabolic pathways of bile pigment derived from both intra- and extrahepatic heme sources, and to design a kinetic model describing the functional compartments and transport channels. The subcellular localization of individual heme and bilirubin compartments will be examined using autoradiography and subcellular fractionation of hepatocytes. Finally, it is proposed to examine the possibility that in the hepatic lobule, quantitative differences may exist between periportal and centrizonal hepatocytes with regard to the rate of bilirubin formation and transport. The information obtained in the course of these studies should contribute to an improved understanding of the regulation of heme and bilirubin metabolism, and have a direct bearing on the hepatic transport and metabolism of xenobiotics and other endogenous organic anions.